The prior art includes various circuits which operate to linearize the output of piezoresistive transducers. Essentially, piezoresistive devices such as transducers will exhibit a varying output voltage upon application of a varying pressure or force. In regard to transducers, in general, it is desirable that the output voltage be linearly related to the applied pressure or force and thus exhibit a straight line characteristic. In practice such devices do not exhibit a straight line characteristic but are non-linear and, for example, the output voltage for a certain range of pressures does not follow the same slope as for lower ranges of pressures. This results in a non-linear transfer characteristic of the transducer.
Techniques exist in the prior art as to how to achieve compensation of such a device. The prior art has many examples of circuits which will increase the voltage applied to the transducer as the pressure increases. In this way the non-linearity is compensated for. Non-linearities can arise for many reasons in piezo-resistive pressure sensors. These include electrical, material and mechanical non-linearities, all of which add to a sensor's overall non-linearity. Material and electrical non-linearities tend to be random and are usually not large. Mechanical non-linearities are generally small as well. However, they can become large when the diaphragm deflection is not small compared to its thickness. This occurs with low pressure transducers or when a high output is needed for other pressure ranges. In these situations the diaphragm does not deflect as much as it would under linear conditions, which therefore leads to a loss in output at greater pressures. This non-linearity is known as a negative non-linearity. The negative non-linearities are a problem for all high accuracy pressure sensors but is especially troublesome when two half bridges are used to make a differential sensor.
Reference is made to U.S. Pat. No. 6,612,179, entitled Method and Apparatus for the Determination of Absolute Pressure and Differential Pressure Therefrom, issued on Sep. 2, 2003 to A. D. Kurtz, one of the inventors herein and assigned to Kulite Semiconductor Products, Inc., the assignee herein. That patent shows a combination absolute and differential pressure sensing device, which includes a plurality of absolute pressure transducers. Each transducer includes a plurality of half-bridge piezoresistive structures and means for selectively coupling at least one of the plurality of half-bridge structures of a first absolute transducer to at least one resistor or half-bridge of another piezoresistive structure. This forms a half-active, full Wheatstone bridge circuit, which can be adapted to measure an absolute pressure. Furthermore, at least one other of the plurality of half-bridge structures is coupled to another half-bridge structure to form a full Wheatstone bridge adapted to measure differential pressure. In this particular arrangement, small changes in the output at high pressures will lead to large differences in output in differential pressure at different line pressures. Namely, the output with the main transducer at 25% and the reference at 0 will be larger than the output with the main transducer at 100% and the reference at 75%. The entire disclosure of U.S. Pat. No. 6,612,179 is hereby incorporated by reference as if being set forth in its entirety herein.
As indicated above, there exist ways to compensate for such a non linearity that use active elements, such as amplifiers and transistors. For an example of such a compensation technique, reference is made to U.S. Pat. No. 4,419,620, entitled Linearizing Circuits for a Semiconductor Pressure Transducer, issued on Dec. 6, 1983 to A. D. Kurtz et al, and assigned to the assignee herein. In that patent there is shown a circuit which linearizes the output of a differential pressure transducer. The circuit includes first and second operational amplifiers, where each amplifier is associated with threshold devices such as diodes, so that one amplifier will be active for one condition of output polarity while the other amplifier will be active for the other condition of output polarity. The conditions can be positive and negative. The output of the amplifiers are connected to a common terminal, which provides a compensating biasing voltage to the transducer. The voltage varies in a “V” shaped characteristic to enable one to achieve a linear output voltage from the transducer for both positive and negative pressure differences. The entire disclosure of U.S. Pat. No. 4,419,620 is hereby incorporated by reference as if being set forth in its entirety herein.
The approach of U.S. Pat. No. 4,419,620 requires the use of active elements such as transistors and diodes. The active elements are subjected to many other problems if they are not properly isolated. These problems are associated as well with changes in temperature and voltage changes, whereby other separate compensating techniques are required for the active devices.
It would be desirable to enable compensation and linearization of a differential sensor device, such as a device including two half-bridges, and to compensate or linearize the device using only simple passive components for high temperature or low voltage applications.